Method and system for fabricating an aspherical orthopaedic prosthetic component

ABSTRACT

A method and apparatus for fabricating an aspherical orthopaedic prosthetic component includes rotating the orthopaedic prosthetic component and operating an electrical discharge machining cutting tool (EDM) to move a wire electrode of the EDM along a number of arcuate cutting paths. A spark is generated between the wire electrode and the orthopaedic prosthetic component to remove a portion of the orthopaedic prosthetic component.

CROSS REFERENCE

Cross reference is made to copending U.S. patent application Ser. No. ______ entitled “Method and Apparatus for Fabricating an Orthopaedic Prosthetic Component” by Gerald W. Beattie, Jr. (Attorney Docket No. 265280-204274, Johnson & Johnson Ref. No. DEP6058), which is assigned to the same assignee as the present application and filed concurrently herewith. The entirety of the above-referenced application is hereby incorporated by reference.

TECHNICAL FIELD

The present disclosure relates generally to methods and systems for fabricating orthopaedic prosthetic components, and more particularly to methods and systems for fabricating orthopaedic prosthetic components having an aspherical surface.

BACKGROUND

Joint arthroplasty is a well-known surgical procedure by which a diseased and/or damaged natural joint is replaced by a prosthetic joint. The orthopaedic prosthetic joint may include a number of components of various shapes and sizes. Some orthopaedic prosthetic components have a spherical shape or otherwise include a curved outer surface. For example, typical “ball-and-socket” prosthetic joints, such as hip and shoulder prosthetic joints, include a head component having a spherical shape, which is configured to be received in a corresponding bearing component which likewise has a spherical shape.

Recently, aspherical designs have been investigated. See, for example, U.S. Pat. No. 6,059,830. In such designs, the outer surface of the head component or the bearing component or both include an aspherical surface—i.e., include multiple arcuate surfaces with different radii.

Orthopaedic prosthetic components having spherical or aspherical outer surfaces are typically manufactured using a multi-step lathing process. Because a lathe is limited in its ability to form curved surfaces with a work piece held in a single orientation, several lathing process steps using different component orientations are required to form the spherical shape or other curved surface. This is compounded in the case of aspherical designs. Additionally, because typical lathes are unable to form curved surfaces within the tolerances required for orthopaedic applications, a subsequent grinding process is required to form the spherical shape or curved surface within the tolerances required.

In addition to lathing processes, other manufacturing processes may be used to fabricate orthopaedic prosthetic components. One such manufacturing process is electrical discharge machining (EDM). Electrical discharge machining may be used to machine materials that are electrically conductive. In electrical discharge machining, a potential difference is generated between an electrode, such as a wire electrode, of the electrical discharge machining tool and the work piece. The potential difference between the electrode and the work piece causes a spark to be generated. The spark erodes a portion of the work piece, and consecutive sparks between the electrode and work piece are used to remove material from the work piece. Because the electrode may also be damaged by the spark, the electrode is continuously replaced. For example, in electrical discharge machining using wire electrodes, the electrode wire is continuously advanced while the work piece is being fabricated.

The work piece may be shaped by moving the electrode and/or the work piece itself. Additionally, spherical shapes may be formed using electrical discharge machining by rotating the work piece while the electrode is moved along an arc. For example, a typical EDM includes a positioning motor to which the work piece is coupled. The positioning motor is used to slowly move the work piece while the wire electrode is moved along an arc. The adaptive feedback control used to cut the work piece is based, in part, on the speed and position of the work piece positioning motor. Typical positioning motors operate at about 1-2 revolutions per minute. At such rotational speeds, fabrication of spherical orthopaedic components using a typical electrical discharge machining process take about four to five days to complete.

SUMMARY

According to one aspect, a method for fabricating an aspherical orthopaedic prosthetic component may include rotating the orthopaedic prosthetic component and operating an electrical discharge machining tool to move a wire electrode of the electrical discharge machining tool along a number of arcuate paths to form an aspherical outer surface in the component.

In some embodiments, the orthopaedic prosthetic component may be rotated at a speed greater than about 100 revolutions-per-minute. For example, in some embodiments, the orthopaedic prosthetic component may be rotated at a speed of about 200 revolutions-per-minute. The orthopaedic prosthetic component may be, for example, a modular head component of a hip or shoulder prosthesis.

A spark is generated between the wire electrode and the orthopaedic prosthetic component to remove a portion of the orthopaedic prosthetic component to form a curved surface of the orthopaedic prosthetic component. For example, the spark may be generated to remove a portion of the orthopaedic prosthetic component to form an aspherical outer surface of the orthopaedic prosthetic component.

In some embodiments, the orthopaedic prosthetic component may be rotated in a first plane and the wire electrode of the electrical discharge machining tool may be moved along arcuate paths defined in a second plane substantially orthogonal to the first plane. Additionally, in some embodiments, the method may include advancing the wire electrode along its arcuate paths without adaptive feedback relating to the rotary speed of the orthopaedic prosthetic component. The method may also include lathing the orthopaedic prosthetic component prior to the rotating step.

According to another aspect, a method for fabricating an orthopaedic prosthetic component, such as a modular head of a hip or shoulder prosthesis, may include rotating the orthopaedic prosthetic component and operating an electrical discharge machining tool to form an aspherical outer surface in the component. A wire electrode of the electrical discharge is advanced along the arcuate cutting paths without adaptive feedback relating to the rotary speed of the orthopaedic prosthetic component. A spark is generated between the wire electrode and the orthopaedic prosthetic component to remove a portion of the orthopaedic prosthetic component to form an aspherical outer surface of the orthopaedic prosthetic component.

In some embodiments, the orthopaedic prosthetic component may be rotated in a first plane and the wire electrode of the electrical discharge machining tool may be moved along cutting paths defined in a second plane substantially orthogonal to the first plane. The method may also include lathing the orthopaedic prosthetic component prior to the rotating step.

BRIEF DESCRIPTION OF THE DRAWINGS

The detailed description particularly refers to the following figures, in which:

FIG. 1 is a simplified block diagram of a system for fabricating an aspherical orthopaedic prosthesis component;

FIG. 2 is a simplified flow diagram of a method for fabricating an aspherical orthopaedic prosthetic component;

FIG. 3 is side perspective view showing the aspherical orthopaedic prosthetic component being fabricated by electrical discharge machining;

FIG. 4 is side elevation view of an orthopaedic prosthetic component after initial lathing but prior to electrical discharge machining;

FIG. 5 is a plan view of the aspherical orthopaedic prosthetic component during fabrication; and

FIG. 6 is a perspective view of the aspherical orthopaedic prosthetic component during fabrication.

DETAILED DESCRIPTION OF THE DRAWINGS

While the concepts of the present disclosure are susceptible to various modifications and alternative forms, specific exemplary embodiments thereof have been shown by way of example in the drawings and will herein be described in detail. It should be understood, however, that there is no intent to limit the concepts of the present disclosure to the particular forms disclosed, but on the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the invention as defined by the appended claims.

Referring to FIG. 1, there is shown a system 10 for fabricating an orthopaedic prosthetic component 12. The system 10 may be used to fabricate orthopaedic prosthetic components 12 having an aspherical shape. For example, the system 10 may be used to fabricate a modular head of a hip prosthesis or shoulder prosthesis having an aspherical shape. The term “aspherical” is used herein to refer to an outer head component surface or an outer bearing component surface that varies slightly from a spherical or hemispherical shape as a result of having been formed by the use of more than one radius. As a result, the term “aspherical” specifically does not cover component designs intended to be spherical, but which have slight imperfections or anomalies therein as a result of the fabrication process since such designs, having been intended to be spherical, are formed by the use of a single radius.

In the illustrative embodiments described herein, the orthopaedic prosthetic components are made of a biocompatible metal, such as cobalt-chrome alloy, although other types of biocompatible metals may also be used. In addition to biocompatible metals, other types of biocompatible materials may also be used provided such materials are electrically conductive.

The system 10 may be used to fabricate an orthopaedic prosthetic component, a near net-shape orthopaedic prosthetic component, or net-shape orthopaedic prosthetic component from a starting component preform that has been previously subjected to an initial lathing operation, by obtaining such a component preform and machining it with an electrical discharge machine. What is meant herein by the term “component preform” is a work piece that has been subjected to an initial machining operation, such as lathing, to produce certain of the features associated with an orthopaedic prosthetic component. For example, a component preform may be a metallic work piece that has a tapered hole defined therein from a previous lathing process, with the remaining features of the orthopaedic prosthetic component to be formed in the work piece by the system 10. What is meant herein by the term “net-shape orthopaedic prosthetic component” is an orthopaedic prosthetic component that is in a shape or condition that is satisfactory for use in a prosthetic implant upon removal of the component from the electrical discharge machine without requiring any additional machining. The term “near net-shape orthopaedic prosthetic component”, on the other hand, is meant herein to define a prosthetic component which requires a small degree of further manipulation, such as grinding or polishing, to produce the final component. What is meant herein by the term “orthopaedic prosthetic component” is an orthopaedic prosthetic component of any type, condition, shape, or configuration. As such, the term “orthopaedic prosthetic component”, amongst others, includes component preforms, net-shape orthopaedic prosthetic components, and near net-shape orthopaedic prosthetic components.

The system 10 includes an electrical discharge machining tool 14 (hereinafter “EDM”). In the illustrative embodiment described herein, the EDM 14 is embodied as a wire electrode electrical discharge machining cutting tool (WEDM). However, in other embodiments, other types of electrical discharge machining tools may be used. The EDM 14 includes a controller 18, a wire electrode positioning motor 20, a wire electrode feeder 22, and a wire electrode 24. The controller 18 is communicatively coupled to the wire electrode position motor 20 via a number of signal paths 26 and to the wire electrode feeder 22 via a number of signal paths 28. The signal paths 26, 28 may be embodied as any number of cables, wires, printed circuit board traces, and/or other interconnects capable of facilitating communication between the controller 18 and the wire electrode position motor 20 and the wire electrode feeder 22, respectively.

The EDM 14 may be embodied as any type of commercially available EDM. However, as will be described below in greater detail, in the illustrative embodiment described herein, modifications are made to such commercially available machines to facilitate higher volume production. One commercially available EDM that may be used as the EDM 14 is a model number AQ327L EDM which is commercially available from Sodick, Incorporated of Schaumburg, Ill.

In the illustrative embodiment described herein, the EDM 14 has been altered from its commercially-available configuration. Specifically, as shown in FIG. 1, the EDM 14 does not utilize adaptive feedback control from the EDM's work piece positioning mechanism. That is, in typical EDM's, the controller utilizes adaptive feedback from the work piece positioning mechanism to control the cutting process. In such embodiments, the adaptive feedback control is configured for use with very slow rotational speeds of the work piece (e.g., 1-2 revolutions per minute) by the work piece positioning motor. However, in the illustrative embodiment described herein, the conventional work piece positioning motor is not utilized in lieu of a relatively high-speed motor 16. Specifically, in the illustrative embodiment, the motor 16 is utilized to rotate the prosthetic orthopaedic component at much higher rotational speeds (e.g., greater than 100 revolutions per minute) relative to those speeds that would be achievable with a conventional, unmodified EDM. The adaptive feedback control relating to the speed at which the work piece is rotated is removed or otherwise disabled in embodiments using such high rotational speeds. Specifically, as diagrammatically shown in FIG. 1, the cutting process is performed without adaptive feedback relating to the speed of the motor 16. In such a case, the speed at which the work piece is rotated (i.e., the speed of the motor 16) is set at a predetermined speed.

The motor 16 is used to rotate the work piece (i.e., the orthopaedic prosthetic component 12) relative to the EDM's cutting wire electrode 24. As described above, commercially available EDM's typically include a work-piece positioning mechanism having a low-speed motor for rotating the work piece at relatively low speeds (1-2 RPM). However, the system 10 of the illustrative embodiment does not utilize such a low-speed motor, but rather has been retrofitted with a high-speed motor to rotate the orthopaedic prosthetic component at speeds in excess of 100 RPM. In the illustrative embodiment described herein, the motor 16 may be embodied as a model number 3R-6.600-VHS rotating spindle which is commercially available from System 3R USA, Incorporated of Elk Grove Village, Ill. The motor 16 has an output shaft 30 which includes a coupling mechanism 32, such as an arbor or a chuck, for coupling the work piece (i.e., the orthopaedic prosthetic component) to the output of the motor 16.

Referring now to FIG. 2, a process 50 for fabricating an aspherical orthopaedic prosthetic component is shown. The process 50 begins with step 52 in which an initial lathing process is performed on a work piece to form an initial component preform. An illustrative component preform is shown in FIG. 4. In this case, the work piece has been lathed into a configuration in which certain of the features of the orthopaedic prosthetic component are present. For example, a tapered bore, such as a Morse tapered bore, has been formed in one side of the orthopaedic prosthetic component. The portion of the component into which the aspherical surface of the head will be formed is also lathed into an initial shape. As shown in FIG. 4, the component preform will be further machined along the phantom lines to form its final form. As will be described below in greater detail, such further machining will be performed with the EDM 14.

Once lathing of the component preform has been completed, the process 50 advances to step 54 during which the orthopaedic prosthetic component (e.g., a work piece in the form of a component preform) is secured to the output mechanism 32 of the rotating motor 16. Specifically, the component is secured to the chuck or arbor of the motor's output shaft 30 thereby allowing the component to be rotated by the motor 16.

As shown in FIG. 2, once the orthopaedic prosthetic component 12 has been secured to the output of the rotating motor 16, machining of the component 12 with the EDM 14 commences. Specifically, in step 56, the component 12 is rotated at a relatively high rotational speed. In the illustrative embodiment described herein, the component is rotated at a speed greater than 100 RPM. In other embodiments, even faster rotational speeds, including those in excess of 200 RPM, may be used.

With the component 12 rotating, operation of the EDM 14 commences. Specifically, as shown in FIGS. 5 and 6, the controller 18 operates the wire position motor 20 to move the wire electrode 24 along a number of predetermined arcuate paths to create a desired aspherical shape of the orthopaedic prosthetic component 12. In the illustrative embodiment described herein, the wire electrode 24 is manipulated orthogonally relative to the wire electrode. In particular, as shown in FIG. 6, the orthopaedic prosthetic component 12 is rotated in one plane, while the wire electrode 24 is moved along a pair of arcuate paths 36, 38 in another plane that is orthogonal to the plane in which the component 12 rotates. The wire electrode 24 may be moved through numerous different paths in numerous different planes to produce the desired shape of the component.

It should be appreciated that during such machining, the wire electrode 34 is guided along the desired paths 36, 38 and, as such, is positioned very closely to the orthopaedic prosthetic component 12, but it does not physically contact the component 12. Rather, sparks are generated between the wire electrode 24 and the orthopaedic prosthetic component 12. A consecutive number of such sparks produce a series of micro-craters on the component 12 thereby removing material along the cutting paths 36, 38 by melting and vaporization. The component 12 is continuously flushed with a dielectric cutting fluid to remove the particles created by such a process.

During such a process, similar micro-craters are formed on the surface of the wire electrode 24, with such debris likewise being flushed away by the cutting fluid. These micro-craters result in the gradual erosion of the wire electrode 24. As such, the wire electrode 24 is constantly replaced by feeding the wire 24 from a spool. In the illustrative embodiment described herein, the wire electrode 24 has a diameter of 0.25 μm, although other wire electrodes of other diameters may be used to fit the needs of a given system.

As shown most clearly in FIG. 5, advancement of the wire electrode 24 along the arcuate cutting paths 36, 38 forms a pair of curved surfaces 40, 42, respectively. The curved surfaces 40, 42 have different radii. Indeed, as shown in FIG. 5, the curved surface 40 has a radius 44 which is different from the radius 46 of the curved surface 42. In the embodiment shown in FIG. 5, the radius 44 is smaller than the radius 46. The radii 44, 46 may originate from a common center point, or, as shown in the illustrative embodiment of FIG. 5, may originate from different center points. It should be appreciated that although the illustrative embodiment is herein described as having two distinct curved surfaces (with two distinct radii), other configurations may be used including those having additional curved surfaces.

The wire electrode 24 may be advanced along the cutting paths 36, 38 in any order which suits the needs of a given design or produces a desired efficiency. For example, the wire electrode 24 may first be operated to cut along the path 36 to form a component preform with a substantially spherical shape, with the desired aspherical shape being introduced into the component 12 by subsequent advancement of the wire electrode 24 along the path 38. It should be appreciated that the wire electrode 24 need not be advanced along the entire length of the paths 36, 38 shown in FIG. 5, but rather along only that portion of the path which causes material to be removed from the component 12. In particular, the phantom lines in FIG. 5 representing the cutting paths 36, 38 are shown as relatively large arc segments for clarity of description. However, in practice, the wire electrode 24 may not need to be advanced along the entirety of the arc segment, but rather only that portion of the arc segment required to make the desired cuts in the rotating component 12.

Conventional EDM wire advancing protocols rely on adaptive feedback to control the cutting process in which, amongst other things, output from the work piece positioning mechanism is utilized. However, such a methodology is not conducive to high-speed machining of the component 12. As such, in step 58, the controller 18 operates the motor 16 essentially in open-loop at a predetermined speed. In other words, the machining process is performed without adaptive feedback relating to the speed of the motor 16. Unexpectedly, it has been found that by doing so, the limitations on high-speed machining imposed by typical closed-loop EDM work-piece-positioning methodologies can be eliminated thereby facilitating high-speed machining of the component 12.

As shown in FIG. 2, once the desired shape and configuration of the orthopaedic prosthetic component 12 has been achieved, the component 12 is removed from the EDM 14. In the case of when the EDM 14 is utilized to produce a net-shape orthopaedic prosthetic component 12, the process 50 ends. However, in the case of when the EDM 14 is used to produce a near net-shape orthopaedic prosthetic component 12, the component 12 is removed from the EDM 14 and the process 50 continues to step 62. In step 62, the near net-shape orthopaedic prosthetic component 12 is subjected to subsequent final machining. Such final machining may take the form of a polishing or fine grinding operation to create the desired final finish of the orthopaedic prosthetic component 12 (i.e., to produce the desired net-shape orthopaedic prosthetic component). Once this final machining is complete, the process 50 ends.

As described herein, the methods and systems of the present disclosure may be used to produce orthopaedic prosthetic components in relatively higher volumes utilizing a technique (i.e., electrical discharge machining) that typically could only be used for relatively lower volumes of manufacturing. As a result, electrical discharge machining may be used to produce custom implants, whereas such a technique typically could only be used for lower volume, slower applications such as prototyping. For example, to produce a modular head component of a hip prosthesis or shoulder prosthesis utilizing electrical discharge machining would typically take several days to complete a single head component. To manufacture saleable custom implants with such throughput times is impractical. However, by utilizing the methods and systems disclosed herein, it has been found that such head components can be produced in as little as a few hours apiece. Indeed, in one case, a head component was produced in eight hours. Such an improvement in throughput times has enabled the use of electrical discharge machining in the manufacture of saleable custom implants.

However, it should be appreciated that if such throughput advantages are not needed or otherwise not sought, the concepts disclosed herein may be used to machine aspherical orthopaedic prosthetic components without the use of the high-speed motor 16. In doing so, adaptive feedback including the position of the work piece could be used to control the cutting process.

While the disclosure has been illustrated and described in detail in the drawings and foregoing description, such an illustration and description is to be considered as exemplary and not restrictive in character, it being understood that only illustrative embodiments have been shown and described and that all changes and modifications that come within the spirit of the disclosure are desired to be protected.

There are a plurality of advantages of the present disclosure arising from the various features of the method and system described herein. It will be noted that alternative embodiments of the method and system of the present disclosure may not include all of the features described yet still benefit from at least some of the advantages of such features. Those of ordinary skill in the art may readily devise their own implementations of the method and system that incorporate one or more of the features of the present invention and fall within the spirit and scope of the present disclosure as defined by the appended claims. 

1. A method for fabricating an aspherical orthopaedic prosthetic component, the method comprising: rotating the orthopaedic prosthetic component, operating an electrical discharge machining tool to move a wire electrode of the electrical discharge machining tool along a first arcuate path to form a first curved surface having a first radius in the orthopaedic prosthetic component, and operating the electrical discharge machining tool to move the wire electrode of the electrical discharge machining tool along a second arcuate path to form a second curved surface having a second radius in the orthopaedic prosthetic component, the second radius being different from the first radius.
 2. The method of claim 1, wherein: operating the electrical discharge machining tool to move a wire electrode of the electrical discharge machining tool along the first arcuate path comprises generating a spark between the wire electrode and the orthopaedic prosthetic component to remove a portion of the orthopaedic prosthetic component along the first curved surface, and operating the electrical discharge machining tool to move a wire electrode of the electrical discharge machining tool along the second arcuate path comprises generating a spark between the wire electrode and the orthopaedic prosthetic component to remove a portion of the orthopaedic prosthetic component along the second curved surface.
 3. The method of claim 1, wherein rotating the orthopaedic prosthetic component comprises rotating the orthopaedic prosthetic component at a speed greater than about 100 revolutions-per-minute.
 4. The method of claim 1, wherein rotating the orthopaedic prosthetic component comprises rotating the orthopaedic prosthetic component at a speed of about 200 revolutions-per-minute.
 5. The method of claim 1, wherein: rotating the orthopaedic prosthetic component comprises rotating the orthopaedic prosthetic component in a first plane, and both the first arcuate path and the second arcuate path along which the wire electrode is advanced are defined in a second plane that is substantially orthogonal to the first plane.
 6. The method of claim 1, wherein operating the electrical discharge machining tool to move a wire electrode of the electrical discharge machining tool along the second arcuate path comprises forming an aspherical outer surface of the orthopaedic prosthetic component.
 7. The method of claim 1, further comprising advancing the wire electrode along both the first arcuate path and the second arcuate path without adaptive feedback relating to the rotary speed of the orthopaedic prosthetic component.
 8. The method of claim 1, further comprising lathing the orthopaedic prosthetic component prior to the rotating step.
 9. The method of claim 1, wherein the orthopaedic prosthetic component is a modular head component of a hip prosthesis.
 10. The method of claim 1, wherein the orthopaedic prosthetic component is a modular head component of a shoulder prosthesis.
 11. The method of claim 1, wherein the center point of the first radius is different than the center point of the second radius.
 12. A method for fabricating an aspherical orthopaedic prosthetic component, the method comprising: rotating the orthopaedic prosthetic component, and operating an electrical discharge machining tool to (i) generate a spark between a wire electrode of the electrical discharge machining tool and the orthopaedic prosthetic component to remove a portion of the orthopaedic prosthetic component, and (ii) advance the wire electrode along a number of arcuate paths to form an aspherical outer surface on the orthopaedic prosthetic component.
 13. The method of claim 12, wherein rotating the orthopaedic prosthetic component comprises rotating the orthopaedic prosthetic component at a speed of about 200 revolutions-per-minute.
 14. The method of claim 12, wherein: rotating the orthopaedic prosthetic component comprises rotating the orthopaedic prosthetic component in a first plane, and operating the electrical discharge machining tool comprises operating the electrical discharge machining tool to move the wire electrode along the number of arcuate paths defined in a second plane substantially orthogonal to the first plane.
 15. The method of claim 12, further comprising advancing the wire electrode along the number of arcuate paths without adaptive feedback relating to the rotary speed of the orthopaedic prosthetic component.
 16. The method of claim 12, wherein the orthopaedic prosthetic component is a modular head component of a hip prosthesis.
 17. The method of claim 12, wherein the orthopaedic prosthetic component is a modular head component of a shoulder prosthesis.
 18. The method of claim 12, wherein: the number of arcuate paths includes a first arcuate path having a first radius and a second arcuate path having a second radius, and the center point of the first radius is different than the center point of the second radius.
 19. The method of claim 12, further comprising lathing the orthopaedic prosthetic component prior to the rotating step.
 20. The method of claim 12, wherein rotating the orthopaedic prosthetic component comprises rotating the orthopaedic prosthetic component at a speed greater than about 100 revolutions-per-minute. 